Pompey are eyeing a move for Burton's Damien McCrory.
The left-sided attacking defender has been identified as a potential solution to strengthening that flank in Kenny Jackett's team.
Pompey have already signed Oliver Hawkins and Stuart O'Keefe during what is developing into a busy deadline day.
McCrory has also emerged as a target as the Blues seek further recruits ahead of the 11pm deadline, including Cardiff's Matty Kennedy.
The 27-year-old has made 196 appearances and scored 10 goals for Burton, while started last weekend's 1-1 draw against Sheffield Wednesday.